blink_gc_plugin: Remove unused member MissingMixinMarker::ast_context_.

Pointed out here:
https://chromium-review.googlesource.com/c/chromium/src/+/1340784/10#message-30c972195a41ceef15c7d9447bb8f75c8ca60e63

Change-Id: Ie420f10a93ab95bfa2b792e8c09ee5b44c246fea
Reviewed-on: https://chromium-review.googlesource.com/c/1394385
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619643}
1 file changed